The sieve pores of old sieve tubes are blocked by a carbohydrate known as

Options

A. Arabinose
B. Raffinose
C. Callose
D. Callus

Answer

To solve the question regarding what carbohydrate blocks the sieve pores of old sieve tubes, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Structure of Sieve Tubes**:
- Sieve tubes are composed of sieve tube members that are tubular structures. They have specialized areas known as sieve plates which contain small perforations.

2. **Identify the Function of Sieve Tubes**:
- Sieve tubes are responsible for the transport of nutrients, particularly sugars, throughout the plant.

3. **Consider the Aging Process**:
- As sieve tubes age, they may become non-functional. This is a natural part of their lifecycle.

4. **Research the Blockage of Sieve Pores**:
- In old or non-functional sieve elements, the perforations in the sieve plates can become blocked to maintain the integrity of the transport system.

5. **Identify the Substance Responsible for the Blockage**:
- The carbohydrate that blocks these sieve pores is known as "callose." Callose is a polysaccharide that accumulates in the sieve pores of old sieve tubes.

6. **Select the Correct Answer**:
- Among the options provided (arabinose, raffinose, callose, callus), the correct answer is "callose."

### Final Answer:
The sieve pores of old sieve tubes are blocked by a carbohydrate known as **callose**.

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